The present invention relates to thermal processing, and in particular apparatus and methods for performing rapid thermal processing of a workpiece by uniformly irradiating the workpiece with short pulses of high-irradiance radiation.
Rapid thermal processing (RTP) involves heating (and subsequently cooling) a substrate (xe2x80x9cworkpiecexe2x80x9d) in order to effectuate a change in the workpiece. There are several industrial applications where RTP of a workpiece is required, such as annealing metals, and forming alloys or electrically activating dopants in a semiconductor. There are several RTP techniques known in the art, ranging from slowly heating the workpiece in a furnace, to exposing the workpiece with flashes of radiation from a flash lamp.
Whether a particular RTP technique is suitable for a given application depends primarily on how the workpiece needs to be heated to perform the desired process. For example, to anneal certain semiconductor wafers in forming certain types of semiconductor device structures, heating the wafer with a hot plate or a radiation lamp up to a high temperature and then cooling the wafer by letting it sit on a cooling plate at room temperature is a viable RTP technique. However, this technique is not suitable for applications where only a small region of the wafer needs to be heated and cooled extremely quickly (e.g., microseconds), because the thermal mass of the wafer simply does not allow for such rapid heating and cooling. While a flash-lamp may be used for such rapid heating and cooling, flash-lamps generally do not have the irradiance and temporal pulse length necessary to deliver the amount of energy to the workpiece to effectuate a change in the workpiece for many semiconductor applications.
An RTP technique that could have great potential where sub-microsecond heating and cooling times are required involves using a pulsed laser, and is referred to here as xe2x80x9claser thermal processing,xe2x80x9d or xe2x80x9cLTP.xe2x80x9d The vast majority of RTP techniques require some minimum level of thermal uniformity at the substrate plane. When using an optical source or a laser to heat the substrate, this thermal uniformity requirement translates into an illumination uniformity requirement. Unfortunately, this requirement has hampered the use of pulse lasers for LTP because the pulsed lasers contemplated to date lack the illumination uniformity and pulse-to-pulse stability necessary to effectively carry out RTP.
Generally, when laser radiation is directed onto a workpiece to be processed (e.g., a wafer), micro and macro irradiance non-uniformities arise. The macro-intensity non-uniformity issue has been addressed through a variety of now-common uniformizing techniques, such as light tunnels, homogenizer rods and xe2x80x9cfly""s-eye-arraysxe2x80x9d. However, micro-intensity non-uniformity caused by the coherent nature of laser light has prevented lasers from becoming common light sources for RTP tools. While excimer lasers have been successfully deployed in industry because they are more xe2x80x9cincoherentxe2x80x9d than most other lasers (such as gas discharge or solid state lasers), they are not suitable for all industrial applications, and in particular LTP, because they lack pulse-to-pulse stability. Other problems with excimer lasers include their large size (xe2x80x9cfootprintxe2x80x9d) and their high maintenance costs.
As mentioned above, LTP has great potential application in the semiconductor industry. The fabrication of integrated electronic circuits involves ion implantation to introduce dopants (N or P type) into a semiconductor (e.g., silicon, germanium, gallium arsenide, or the like) substrate to change its conductivity. Generally this procedure is used in implanting source or drains of a MOSFET transistor or base, emitter, collectors of BIPOLAR transistors, the cathode of diodes, a resistive region element, or even as a capacitor plate. In short, there are many reasons why it is desirable to change the conductivity of a semiconductor substrate. The implantation of the dopant atoms breaks the chemical bonds of the crystalline substrate where they are implanted, and in some cases can render a region amorphous, that is, break the crystalline lattice of the region.
To obtain good electrical performance of the electronic components defined by the implantation, the implanted regions must be annealed. The annealing process takes the regions that were previously made amorphous and recreates a more crystalline structure. Also, the dopants need to be xe2x80x9cactivatedxe2x80x9d by incorporating these atoms into the crystalline lattice of the semiconductor substrate. This requires providing a relatively large amount of thermal energy to the region in a short of amount of time, and then rapidly cooling the region to terminate the thermal process.
A successful, robust LTP apparatus preferably satisfies ten design requirements. The first is that the apparatus be fully automated and include remote wafer handling, so that many substrates (xe2x80x9cworkpiecesxe2x80x9d) can be processed without the need for human intervention. The second is that the apparatus expose full die-by-die fields, i.e., no exposure of a partial field. The third is that the apparatus provide sufficient irradiance per pulse to accomplish the goals of LTP, such as dopant activation or thermal annealing, which require irradiance levels of between 0.1 J/cm2 to 1.0 J/cm2 per pulse. The fourth is that illumination uniformity (both macro- and micro-uniformity) over the exposure field be within xc2x15%, so that the corresponding thermal uniformity is equally uniform. The fifth requirement is that the pulse-to-pulse energy stability (repeatability) of the laser be within xc2x15% (and preferably vary only by nanoseconds for sub-microsecond pulses) so that the results from field to field are repeatable. The sixth is that each die (i.e., workpiece field) on the workpiece be aligned to the exposure field to an accuracy within xc2x150 microns (within the non-active KERF design region) so that proper exposure is contained within each field. The seventh is that the illumination fall-off at the edge of the exposure field be very sharp, i.e., a resolution of less than 50 microns, so that there is no exposure of adjacent fields on the workpiece. The eighth is that the field size should be definable from 1 mmxc3x971 mm up to 22 mmxc3x9722 mm to allow for the variety of field sizes for which the LTP apparatus could be used. The ninth is that the apparatus be programmable to deliver energy from 0.1 J/cm2 to 1 J/cm2. The tenth is that the apparatus have diagnostic ability to monitor certain key parameters associated with the LTP process, such as whether the workpiece has melted, the amount of transmitted energy, the amount of reflected energy from the workpiece, and the beam profile.
There are many prior art illumination apparatus that provide uniform illumination, but that do not meet the above-identified requirements. For example, U.S. Pat. No. 5,059,013, entitled xe2x80x9cIllumination System to Produce Self-luminous Light Beam of Selected Cross-section, Uniform Intensity and Selected Numerical Aperture,xe2x80x9d discloses an illumination system which produces a light beam of selected cross-section shape and uniform intensity, which emits self-luminously into a selected numerical aperture, by: providing a non-uniform, non-self-luminous laser light beam; configuring the beam to eliminate the non-uniformity""s near the beam periphery; providing the semi-uniform light beam to a light gate; providing also a lamp light beam with optics and infra-red trap; gating selectively the laser light beam or the lamp light beam to a light beam characterization subsystem; configuring the selected semi-shaped semi-uniform non-self-luminous light beam to provide a selected shaped semi-uniform non-self-luminous light beam; focusing the selected shaped semi-uniform non-self-luminous light beam, with a focal length related to the selected numerical aperture, onto the input plane of a total-internally-reflective beam-shaper-uniformizer, causing multiple reflections within the uniformizer and overlapping of different parts of the beam with one another, the length of the beam-shaper-uniformizer being related to the selected intensity uniformity, resulting in a selected shaped uniform semi-self-luminous light beam at the exit plane; diffusing the light beam; passing the selected shaped uniform self-luminous light beam through a collection lens to convey the beam with the selected numerical aperture toward an application subsystem. Unfortunately, this illumination system is fairly complex and it does not deal with the issue of micro-non-uniformities resulting from laser-coherence effects within the totally-internally-reflecting homogenizer rod. This violates the forth requirement which states a macro- and micro-uniformity requirement of  less than xc2x15%.
U.S. Pat. No. 5,357,365, entitled xe2x80x9cLaser Beam Irradiating Apparatus Enabling Uniform Laser Annealing,xe2x80x9d describes a laser beam irradiating apparatus capable of laser annealing with high precision and uniformity over the entire surface of a sample. Luminous flux of the laser beam output from a laser source is expanded by a beam expander. The power of the laser beam, which has passed through the beam expander, is adjusted by a half-wave plate of synthetic quartz and a polarizing prism of synthetic quartz. The laser beam emitted from polarizing prism is guided to a prescribed position by mirrors, and swung in the direction of the X-axis by an X-axis rotation mirror. The laser beam reflected from X-axis rotation mirror has its diameter reduced by an f-xcex8 lens to have a prescribed beam spot diameter on the surface of a silicon wafer, and laser beam scanning is carried out at a constant speed. Since half-wave plates and the polarizing prisms are formed of synthetic quartz, thermal deformation of optical components caused by continuous irradiation of a laser beam can be suppressed, beam profile of the laser beam can be stabilized, therefore highly uniform and highly precise laser annealing becomes possible. Unfortunately, this illumination system is fairly complex and is reduced in size at the workpiece. That is, only a small area is exposed and is scanned across the workpiece. This approach is inconsistent with the second requirement of providing a full field exposure.
The present invention relates to thermal processing, and in particular apparatus and methods for performing rapid thermal processing of a workpiece by uniformly irradiating the workpiece with short pulses of high-irradiance radiation.
A first aspect of the present invention is an LTP apparatus for processing a workpiece having one or more workpiece fields. The apparatus comprises a pulsed, solid state laser light source having more than 1000 spatial modes and capable of emitting one or more pulses of radiation with a temporal pulse length between 1 nanosecond and 1 microsecond, a workpiece stage for supporting the workpiece, and an illumination optical system having an exposure field, the system being arranged between the laser light source and the workpiece holder so as to illuminate within the exposure field at least one of the one or more workpiece fields with the one or more pulses of radiation having an irradiance uniformity of less than xc2x15%.
A second aspect of the present invention described above is the apparatus, wherein the illumination optical system includes, in order along an optical axis from the laser light source, a beam transport system, a diffuser, an optical integrator, a variable aperture stop for defining the size of the exposure field, and an imaging optical system for forming the exposure field.
A third aspect of the present invention is a method of performing laser thermal processing of a workpiece over an exposure field. The method comprises the steps of first, aligning the workpiece to the exposure field, then providing one or more substantially incoherent radiation pulses having an irradiance from a laser light source having greater than 1000 spatial modes and capable of emitting radiation pulses having a temporal pulse length of between 1 nanosecond and 1 microsecond with a pulse to pulse repeatability of less than xc2x15%, uniformizing the radiation pulse such that the radiation pulse vary in uniformity by less than xc2x15% over the exposure field, and exposing the workpiece with the one or more radiation pulses over the exposure field.